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Attic ventilation education for clients

Jeff Remas

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Hi Chris,

Well, without reading the data at your link, Chris, I can tell you that they're not a very smart option in my climate at all. So far, I think the best option that I've seen in this climate is full-length ridge vents combined with eave vents under the entire length of the eaves and without gable end vents. Around here, those attic fans tend to pull moisture-laden air into the attic and cause mold to develop on the sides of rafters facing away from the fan.



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I know, I watched it. Most of it looked very good, I should have added that in my original post. I'm just seeing a plethora of power vents being added in these parts and homeowners wondering why their utility bills are higher after the install. Too many__________ (inspectors, contractors, homeowners...fill in the blank) think if a little ventilation is good, then a lot is better.

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Originally posted by Jeff Remas

I added this to my website. It is from the Airvent company.

I have a flash version that has to be downloaded or this one which is a .swf version.

Does it work for you guys?

http://www.remasinspections.com/Downloa ... II_2k4.swf

It worked fine in my computer.

Great idea.

- Jim Katen, Oregon

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Thanks Jeff & others. I never thought about ridge vent media clogging up. Re maintenance, Jeff, would you have the gutter cleaning guy blowout the ridge vents with forced air, or what? Any ideas of manufacturers' recs on that?

A building science center at GA tech doesn't like powered vents, especially for retrofits, as they pull conditioned air out of the living spaces. Long ago I specified turbines for a housing rehab job, and the little old lady resident had the contractor switch them for turtlebacks because they created drafts in the house.

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The other problem with power vents is that they can cause gas appliance flues (that pass through the attic) to backdraft from depressurization, which can present a CO problem.


7. Leaks in the wrong parts of the house: wind blowing against a house creates high pressure on the windward side and low pressure on the lee. So, even a leaky house can become seriously depressurized if the predominate leaks are on the downwind side. These leaks could be intentional. Fresh-air inlets for a fireplace or furnace our even a dryer exhaust can allow wind to pull air from the house. And any house can become depressurized if there's a window open on the downwind side.

Leaks at the wall-ceiling junction are also likely to encourage depressurization, since they help the house to act like a good chimney. This effect is more pronounced in a multi-story house, simply because it's taller. Likewise, open windows on the second story (and particularly downwind) may further increase depressurization.

Gary Nelson also noticed an interesting phenomenon in three Minnesota houses he's investigated. All three were fitted with power attic ventilators in attempts to solve moisture problems in the insulation. According to Nelson, the moisture was coming from the house through holes in the ceiling, so exhausting air from the attic only increased leakage rates. Not surprisingly, increasing the leakage rate increased depressurization, and all three houses had backdrafting problems.

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